Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of the positive silver images, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith, a soluble silver complex is obtained by reaction of a silver halide solvent with the unexposed and undeveloped silver halide of said emulsion. Preferably, the photosensitive silver halide emulsion is developed with a processing composition in a viscous condition which is spread between the photosensitive element comprising the silver halide emulsion and a print-receiving element comprising, preferably, a suitable silver precipitating layer. The processing composition effects development of the latent image in the emulsion and, substantially contemporaneously therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate, with undeveloped silver halide. This soluble silver complex is, at least in part, transported in the direction of the print-receiving element and the silver thereof is largely precipitated in the silver precipitating element to form a positive image thereon. Procedures of this description are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. See, also, Edwin H. Land, One Step Photography, Photographic Journal, Section A, pp. 7-15, January 1950.
Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color, such as red or green or blue, and by viewing the reversed or positive silver image formed by transfer to a transparent print-receiving element through the same or a similar screen which is suitably registered with the reversed positive image carried by the print-receiving layer.
As examples of suitable film structures for employment in additive color photography, mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,427; 3,615,428; 3,615,429; 3,615,426; and 3,894,871.
The image-receiving elements of the present invention are particularly suited for use in diffusion transfer film units wherein there is contained a positive transfer image and a negative silver image, the two images being in separate layers on a common, transparent support and viewed as a single, positive image. Such positive images may be referred to for convenience as "integral positive-negative images", and more particularly as "integral positive-negative transparencies." Examples of film units which provide such integral positive-negative transparencies are set forth, for example, in the above-indicated U.S. Pat. Nos. 3,536,488; 3,894,871; 3,615,426; 3,615,427; 3,615,428; and 3,615,429.
In general, silver precipitating nuclei comprise a specific class of adjuncts well known in the art as adapted to effect catalytic reduction of solubilized silver halide specifically including heavy metals and heavy metal compounds such as the metals of Groups IB, IIB, IVA, VIA and VIII and the reaction products of Groups IB, IIB, IVA and VIII metals with elements of Group VIA.
Widely used as silver precipitating agents have been those disclosed in U.S. Pat. No. 2,698,237 and specifically the metallic sulfides and selenides, there detailed, these terms being understood to include the selenosulfides, the polysulfides, and the polyselenides. It has been preferred to employ sulfides whose solubility products in an aqueous medium at approximately 20.degree. C. vary between 10.sup.-23 and 10.sup.-49, and especially the salts of zinc. Also particularly suitable as precipitating agents heavy metals such as silver, gold, platinum, palladium, etc., and in this category the noble metals illustrated are preferred and are generally provided in a silica matrix as colloidal particles.
The nucleation layers in the art have generally been found to be in excess of the 1 micron range (about 93 mgs/ft.sup.2). See, for example, U.S. Pat. Nos. 3,220,837 and 3,709,687.
U.S. Pat. No. 3,647,440, issued Mar. 7, 1972 discloses receiving layers comprising finely divided non-silver noble metal nuclei obtained by reducing a noble metal salt in the presence of a colloid or binder material with a reducing agent having a standard potential more negative than -0.30. It is the thrust of the patent that a reducing agent having a standard potential more negative than -0.30 must be used in order to obtain nuclei of a specific, usable size range. It is further illustrated that stannous chloride, which does not fall within the standard potential range, does not produce useful nuclei. The binder materials disclosed include gelatin, polyvinyl pyrrolidone, polymeric latices such a copoly (2-chloroethylmethacrylate-acrylic acid), a mixture of polyvinyl alcohol and the interpolymer of n-butyl acrylate, 3-acryloyloxypropane-1-sulfonic acid, sodium salt and 2-acetoacetoxyethyl methacrylate, polyethylene latex, and colloidal silica. THe amount of colloid binder employed ranges from about 5 to 500 mgs/ft.sup.2 with the nuclei ranging from 1 to 200 micrograms/ft.sup.2.
A novel silver precipitating nuclei system has now been found which possesses advantages not found in the prior art, which system has particular value in silver transfer color images.